What is a CSS BOX MODEL?

Suppose we already have created a div tag and we want to find the box model.
You have to select chrome developer tool and click on div tag and after click on computed section and you can see the css box model.

#wel

{

background-color: blue; width: 400px; border:5px solid black; padding: 20px; margin: 20px;

}

body

{

display: grid; justify-content: center; align-items: center;

}